1. Field of the Invention
The present invention relates to an optical information reproduction apparatus for reproducing recorded information from an optical recording medium.
2. Description of Related Art
In order to protect its recording surface, an optical disk used as an optical recording medium is provided with, a transmissive substrate with a predetermined thickness in such a way as to cover the recording surface. An optical pickup used as an information reading means reads recorded information from the optical disk in accordance with the amount of reflected light when a reading beam of light is irradiated onto the recording surface through the transmissive substrate.
However, in manufacturing, it is difficult to form the transmissive substrates of all optical disks with a prescribed thickness. Usually, a thickness error of several tens of micrometers occurs. Therefore, the thickness error of the transmissive substrates produces spherical aberration. The production of the spherical aberration sometimes lowers markedly the amplitude level of an information reading signal or a tracking error signal, and, disadvantageously, it lowers information reading accuracy.
The present invention has been made to solve the problem, and an object of the present invention is to provide an optical information reproduction apparatus capable of improving information reading accuracy by correcting spherical aberration even if the spherical aberration is produced by a thickness error of the transmissive substrate of an optical disk.
The present invention is characterized in that an optical information reproduction apparatus for reproducing recorded information from an optical recording medium comprises an optical system that includes a laser generating element for generating a laser beam, an objective lens for condensing the laser beam on a recording surface of the optical recording medium, an objective lens transfer means for transferring the objective lens on a focus adjusting orbit, and a photodetector for obtaining a photoelectric conversion signal by applying photoelectric conversion to reflected light from the optical recording medium; a means for generating a reading signal and a tracking error signal in accordance with the photoelectric conversion signal; a means for controlling the objective lens transfer means so as to transfer the objective lens on the focus adjusting orbit so that the amplitude level of the reading signal reaches a maximum and to retain the objective lens at a position at the maximum level; a spherical aberration correction means for correcting spherical aberration occurring in the optical system to an extent corresponding to the amount of correction; and a means for supplying, to the spherical aberration correction means, a final amount of spherical aberration correction where the amplitude level of the tracking error signal reaches a maximum while changing the amount of correction and detecting the amplitude level of the tracking error signal.